An oil mud such as crude oil sludge has high energy. For this reason, the oil mud has been expected to be effectively used as fuel, instead of being incinerated or landfilled as an industrial waste.
However, an oil mud may have a high viscosity, or lack flowability at normal temperature, or the solid content may precipitate, separate, and be firmly fixed. This is why the oil mud has poor handleability in transportation via pipelines and is difficult to handle as a fuel.
Accordingly, in Europe, solid fuel has been produced in recent years by mixing an oil mud with sawdust, rather than using the oil mud alone. This solid fuel has been suggested for use as a substitution fuel for cement kilns. In Japan, similar methods for producing solid fuel or methods for incinerating oil-containing sludge have also been suggested (for example, Japanese Patent Applications Laid-open No. S54-39401 and 2002-323213).
However, when a viscous oil mud such as crude oil sludge is mixed with a biomass such as sawdust, a combustible gas volatilizes, creating a risk of explosion or inflammation.
Thus, because a mixing apparatus has a high degree of hermeticity, when an oil mud and a biomass are loaded into the mixing apparatus, air in an amount at least equal to the volume of the load is discharged from the mixing apparatus. Because the effluent gas from the mixing apparatus contains a large amount of dust, the effluent gas is sucked into a bag filter or the like so as to prevent the effluent gas from leaking to the outside of the apparatus. As a result of such suction, new air (oxygen content 21%) is introduced into the mixing apparatus, and a state with a high oxygen concentration is assumed inside the mixing apparatus. Because the lower explosive limit concentration of oxygen for most combustible gases is about 10%, where a combustible gas volatilizes inside the mixing apparatus, there is a high risk of explosion or inflammation due to a sufficient amount of oxygen.
Accordingly, a method for preventing explosion and inflammation has been employed by which the concentration of oxygen is reduced (to 8% or less) by blowing an inactive gas such as nitrogen or carbon dioxide into the mixing apparatus. However, such method requires that either the equipment for producing the inactive gas such as nitrogen or carbon dioxide be installed or the inactive gas be purchased and, therefore, the method cannot be implemented at a low cost. Another problem associated with this method is that the effluent gas containing the inactive gas and combustible gas has to be subjected to post-treatment.
When a substance from which a combustible gas volatilizes, such as a solid fuel that is the above-described oil mud or a mixture of an oil mud and a biomass, is stored in a container or apparatus with a high degree of hermeticity, there is also a risk of combustion or inflammation and a problem similar to the above-described problem is encountered.